In recent years, much research and development effort has been expended in the development of fiber-optic communications systems in which digital or analog signals are transmitted through glass fibers from one point to another using radiation at optical frequencies. The use of fiber-optic communications systems has been and is being explored by telephone companies to transmit telephone calls over long distances, and by other companies that are involved in local data communications for transmitting large amounts of data rapidly among parts of a data processing system or among various data processing systems in a common location.
A primary benefit from the use of optical waves as the information carrying medium is the fact that more information, for example, bits of data or telephone messages, can be carried by the higher frequency optical wave in a shorter period of time than would be possible over the portions of the electromagnetic spectrum that have heretofore been used to transfer information.
However, optical communications systems present numerous problems that are not presented by the prior information transfer arrangements. For example, the use of light waves does not permit a direct electrical connection between the transmitter and the receiver as in the prior arrangements, necessitating new receivers and detectors to change the light waves to electrical signals.
The circuit elements that are most widely used to receive optic signals are photodiodes and phototransistors, which operate in a manner very similar to the photodiode, at least as far as the emitter-base junction is concerned. In a typical application, the photodiode is reverse biased so as to normally not conduct. The photons comprising the light wave are directed to the photodiode's p-n junction, which causes a burst of current to flow through the junction.
In a transmission of digital data, the timed variation in the number of photons received can indicate the receipt of a bit of digital data. In the analog situation, the number of photons received can vary according to the amplitude of an amplitude-modulated analog signal which would be received by the photodiode as varying amounts of current therethrough.
In either the digital or analog situation, since the diode is reverse-biased, the junction of the photodiode act as a capacitor, and the capacitive effects thereof must be taken into account in designing the receiver circuit. The ease or difficulty of coupling an optic fiber to the photodiode or phototransistor is directly related to the area of the p-n junction; the larger the area of the junction, the more easily is the optic fiber coupled to the photodiode. However, with a larger junction area, the capacitance of the diode junction is also higher. The input capacitance to the first amplifying stage, including the capacitance of the photodiode junction, effects the bandwidth-gain product that is required of the subsequent amplifier stage. This product is the product of the gain of the amplifier stage, multiplied by the bandwidth of the amplifier. The gain-bandwidth product is of particular importance in situations where there are no additional stages of amplification between the output of the receiver and the input of a next stage, such as a comparator, which may be used in digital reception. Additional gain stages, or amplifiers having a higher gain-bandwidth product, can be employed; however, these stages or amplifiers would add to the cost and complexity of the receiver.
A further aspect of the photodiodes that are used as detectors in fiber-optic communications systems is that they are basically current sources, rather than voltage sources. That is, the receipt of photons results in a direct variation in flow of current through the photodiode. However, it is generally more desirable at some point in a receiver circuit to work with signal voltage variations rather than signal current variations.
It is therefore an object of the invention to provide a new and improved preamplifier for use with a detector in communications systems having a capacitive signal input source.
It is yet another object of the invention to provide a new and improved preamplifier for use with an optical signal detector in communications systems in which information is transferred using radiation at optical frequencies.
It is yet another object of the invention to provide a new and improved preamplifier for use with a detector that detects and receives radiation, including electromagnetic radiation and nuclear radiation, the detector having a substantial capacitive effect upon the input to the circuit.
It is a further object of the invention to provide a new preamplifier of the type described that converts a current signal to a voltage signal.